Television receiver



Jan. 6, 1942'. p. J. FE WINGS ETAL 2,268,811

' TELEVISION RECEIVER Filed Au 5, .1938

- INVENTORS DAV/D JOHN FEW/N65 ROLANDJUHN KEMP 7+4 4 m-C/L/ ATTORNEYPatented Jan. 6, 1942 UNITED STAT ES OFFICE azsasu 'i 1 TELEVISION mommaDavid John Rowing s. and Roland li mjxemp, Chelmsford, Essex, England,assignors to Radio Corporation of America, a corporation of IlelawareApplication August '5, 1938, Sam-N; 223,148

7 In GreatBritain August 25, 1937 2' Claims. (o1. 178-73) ,1

' if it besought to applyan ordinary A. v.0, system as well known inbroadcast sound programme receiver practice, to a television receiver,there arises a serious defect which is not at first sight obvious. Insuch an ordinary A. V. C. system thegain control is efiected byrectifying the also to destroy desired variations corresponding toaverage picture brightness variations. The main object of this inventionis to'avoid this defect.

According to this invention a television receiver is provided with an A.V. C. system which is responsive not to variations in general incomingcarrier level but to variations in a predetermined selected signalcomponent which, as transmitted, is of substantially constant level. Asignal component available for selection and which is transmitted at asubstantially constant chroniz ing signal such as developed in the finaldetector of a receiving...device.

I Figure 2 shows the controlling wave form and the wave formsdevelopedby the A. V. C. device of. Figure 3.

Figure 3 is a diagram of the circuit A. V. C. ,device.

- Referring (to the drawing, synchronizing sig- I of the nals areseparated out by any knownapparatus (not shown) from the total signalsreceived and then, ifgrequired, amplified. .Thenature of the receivedsignals received from a transmitter which transmits, signals includingaD. C. component representative of general picture brightness is"represented conventionally in Figural. Here three scan lines are.representedatl, II,

and III. I is a dull illumination picture line which has been subjectedto fading; II is the separation out of the synchronizing signals willlevel in most present day television transmitters is provided by thesynchronizing pulses (either the line synchronizing or the framingsynchronizing pulses, or both) these pulses being usually maintained, astransmitted, at about 30% of the peak value of the carrier, astransmitted. In carrying out this invention the selected signalcomponent, after separation at the receiver, is utilized to provide again controlling uni-directional potential which is employed in anyknown convenient way, e. g. as gain controlling grid bias, to controlthe gain of a carrier stage or stages of the receiver.

The invention is illustrated in and further explained in connection withthe accompanying graphical and diagrammatic drawing which relates to oneway of carrying out the invention.

Figure 1 shows a composite picture and-synbe, as shown conventionally ata in Figure 2, a

, series of unidirectional pulses of a polarity which will depend uponthe method of selection employed and of an amplitude which will be (inthe absence'of fading) constant. It is arranged that these pulses are inthe positive direction as shown in Figure 2 at a. Since these pulseswill be of relatively short duration as compared to the intervalsbetween them, direct rectification and smoothing to produce the actualgain controlling potentialis not easy and it is therefore much to bepreferred to precede rectification by a wave shape transformingoperation which changes the pulse wave form into a form more convenientfor rectification. One way of doing this is shown in Figure 3. Here thepulses are applied, for example through a coupling condenser I, to thegrid 2 of a valve 3, e. g. a triode whose anode-cathode circuit includesa condenser 4 and whose anode receives positive potential through aresistance 5. The cathode leg contains the usual capacity shuntedself-bias resistance combination 6, I, and the grid receives cut-oilbias through a grid resistance 8. The arrangement is such that duringeach interval between pulses the condenser 4 charges up through theresistance 5, the rate and law of charging depending upon the value ofthis resistance 5, the value of this potential and the size of thecondenser 4. Rectilinearity of charging. is not an important desideratumand the preferred arrangement is that in which the condenser 4 becomescharged substantially to the in Figure 2, the sawtooth having flattenedtops.

If fading occurs and the pulses accordingly become of less amplitudethere will obviously be a less complete discharge of the condenser 4 ateach pulse and the output wave. form will be, as

shown at c in Figure 2, of reduced amplitude with longer flat tops tothe sawtooth. Such a modified sawtooth wave form is quite convenient forrectification. For example, and again as shown in Figure 3, this waveform may be applied through a coupling condenser 9 to a rectifier I0 andwhose output, after smoothing in the usual way, is applied via lead IIas gain controlling bias to the grid or grids of a variable-p. carrieramplifier valve or valves -(not shown) in the receiver. Obviously, anyconvenient form of rectifier may be used; for example, if a double diodeis used the said double diode may be constituted by the diode section ofa double-diodetriode, the triode section of which constitutes the valve3.

Either the line or the frame synchronizing signals may be selected andutilized for the purposes of this invention. Selection of the linesignals presents the advantage of involving less elaborate smoothing(since the line frequency is much higher than the frame frequency) butselection of the frame signals offers the advantage of ease ofseparation.

What we claim is:

1. In an automatic volume control system for radiant energy receiverswherein recurrent signals of a predeterminable amplitude aretransmitted, a thermionic vacuum tube having anode, cathode and at leastone control electrode, means for impressing said recurrent wave onto acontrol electrode of said thermionic tube, means for storing energyconnected in the anodecathode path of said tube, means for storingenergy in said energy storage means during intervals when said tube isin a non-conductive state, and discharging said energy when the waveform impressed onto the control electrode of said tube renders the tubeconducting, said discharge being caused by said signals and proportionalto the amplitude thereof, thus forming a serrated wave form, means forrectifying said wave form to develop a control potential, and means toutilize 'said control potential for automatic volume control purposes.

2.-I'n a television receiver wherein signals of predetermined amplitudeare received intermittently, indicating the level of the transmittedcarrier, the method of developing automatic volume-control signals whichcomprises the steps of storing electrical energy, discharging saidenergy by said signals, and controlling the amount of discharge by theamplitude of said signals, rectifying saidstored energy and utilizingthe rectified energy for automatic-volume-control purposes. I DAVID JOHNFEWDIGS.

ROLAND JOHN KEMP.

